Use /usr/bin/env CMD instead of hardcoded paths

This fixes commands that currently rely on hardcoded paths and break
on systems like NixOS that don't have all binaries in /usr/bin etc.

lib/vagrant-lxc/provider.rb

@@ -36,7 +36,7 @@ module Vagrant
 
       def ensure_lxc_installed!
         begin
-          sudo_wrapper.run("/usr/bin/which", "lxc-create")
+          sudo_wrapper.run("which", "lxc-create")
         rescue Vagrant::LXC::Errors::ExecuteError
           raise Errors::LxcNotInstalled
         end

lib/vagrant-lxc/sudo_wrapper.rb

@@ -14,7 +14,7 @@ module Vagrant
       def run(*command)
         options = command.last.is_a?(Hash) ? command.last : {}
         command.unshift @wrapper_path if @wrapper_path && !options[:no_wrapper]
-        execute *(['sudo'] + command)
+        execute *(['sudo', '/usr/bin/env'] + command)
       end
 
       private